1. Field of the Invention
This invention relates to the recovery of crude oil and more particularly, to a stable micelle solution composition for recovering crude oil which can form a homogeneous micro emulsion over a wide range of salt concentration of an aqueous inorganic salt solution.
2. Description of the Prior Art
A micro emulsion flooding to which attention has been recently paid as one of effective processes of recovering crude oil from oil reservoirs in strata is a method utilizing micelle solutions. In the method, hydrocarbons such as a compounded oil, water, and surface active agents are mixed to form a micro emulsion and this emulsion is charged into oil reservoir to cause the interfacial tension to be lowered between crude oil and water. As a result, the crude oil becomes soluble and thus ready for recovery. This method has the advantage of high recovery efficiency, so that many studies have been made in recent years.
In the micro emulsion flooding, it is the usual practice to use petroleum sulfonates as the surface active agent. On the other hand, for recovery of crude oil in oil fields, sea water or brine containing inorganic salts mainly composed of sodium chloride substantially at the same level as sea water is frequently used, so that there is a demand of micelle solution compositions which are stable against highly concentrated salt solutions. Several methods have been proposed in which petroleum sulfonates are used with or without addition of other types of surface active agents or other aids.
However, in case where a micelle solution is prepared using brine such as salt-containing sea water as injection water, micro emulsion is formed only at a certain salt concentration when petroleum sulfonate is used. Accordingly, there has been adopted a method in which petroleum sulfonates of different average molecular weights are mixed depending on the salt concentration of brine, i.e. a mixture of petroleum sulfonates suitable for a given concentration of salt is provided. However, this method is disadvantageous in that the range of mixing of the petroleum sulfonates which should be varied depending on the concentration of salt in brine is very narrow. Rather, there arises the problem that it is necessary to prepare a brine having a given salt concentration suitable for an employed petroleum sulfonate by adding salt to the brine to adjust its salt concentration. It has been frequently experienced that many oil fields dictate use of sea water or oil reservoir water of high salt concentration. Accordingly, it is highly demanded to develop micelle solution compositions which can readily form micro-emulsions stable against various types of brines whose salt concentration is from as small as about 0.01% to higher than 20% and which cover a wide range of optimum salt concentrations.